linux-kernel - Re: [linux-next:master] [printk] 8fc5f5fc7f: WARNING:at_kernel/printk/printk.c:#console_flush

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Date:   Wed, 30 Nov 2022 19:12:27 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     kernel test robot <yujie.liu@...el.com>
Cc:     John Ogness <john.ogness@...utronix.de>, oe-lkp@...ts.linux.dev,
        lkp@...el.com, Thomas Gleixner <tglx@...utronix.de>,
        Linux Memory Management List <linux-mm@...ck.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [linux-next:master] [printk] 8fc5f5fc7f:
 WARNING:at_kernel/printk/printk.c:#console_flush_all

On Thu 2022-12-01 00:57:06, kernel test robot wrote:
> Greeting,
> 
> FYI, we noticed WARNING:at_kernel/printk/printk.c:#console_flush_all due to commit (built with clang-14):
> 
> commit: 8fc5f5fc7f52a733fcc8b3939d172b9248e63871 ("printk: introduce console_list_lock")
> https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> 
> in testcase: boot
> 
> on test machine: qemu-system-x86_64 -enable-kvm -cpu SandyBridge -smp 2 -m 16G
> 
> caused below changes (please refer to attached dmesg/kmsg for entire log/backtrace):
> 
> 
> [    0.000000][    T0] ------------[ cut here ]------------
> [ 0.000000][ T0] WARNING: CPU: 0 PID: 0 at kernel/printk/printk.c:116 console_flush_all (printk.c:?) 

When I checked out the commit 8fc5f5fc7f52a733fcc8b3939d172b9248e63871
("printk: introduce console_list_lock") then
kernel/printk/printk.c:116 is:

114	 void lockdep_assert_console_list_lock_held(void)
115	{
116		lockdep_assert_held(&console_mutex);
117	}
118	EXPORT_SYMBOL(lockdep_assert_console_list_lock_held);

It is used in

#define for_each_console(con)						\
	lockdep_assert_console_list_lock_held();			\
	hlist_for_each_entry(con, &console_list, node)

that is used in

static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handover)
{
[...]
		for_each_console(con) {

that is called without console_list_lock().

Hmm, we could not take console_list_lock() here. It would violate lock
ordering. console_flush_all() is called under console_lock(). And
console_list_lock() is taken outside of console_lock() in register_console().

Fortunately, we do not have to do it. for_each_console() is still safe
under console_lock() at this stage. And later patches will switch
it to for_each_console_srcu() that will not require
console_list_lock().

So, the solution is to remove the assert in for_each_console() in
the commit 8fc5f5fc7f52a733fcc8b3939d172b9248e63871 ("printk:
introduce console_list_lock").

We need to add the assert later when removing the console_lock()
synchronization of @console_list in register_console().

Best Regards,
Petr